Electrochemical processing of inorganic chemicals in electrolytic diaphragm cells for the production of other inorganic materials is well known. The electrolytic cell typically comprises an anolyte compartment containing an anode, a catholyte compartment containing a cathode, and a microporous diaphragm that separates the anolyte compartment from the catholyte compartment. Diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or an alkaline electrolyte from an acidic electrolyte.
A non-limiting example of a diaphragm electrolytic cell is the electrolytic diaphragm cell used for the electrolysis of aqueous alkali metal halide solutions, e.g., aqueous sodium chloride solutions. In such an electrolytic cell, the diaphragm is usually a portion of the cathode assembly, e.g., it is placed adjacent to or directly on the cathode, and separates the acidic liquid anolyte from the alkaline catholyte liquor. In the process of electrolyzing an aqueous alkali metal halide solution (brine) in the electrolytic diaphragm cell, liquid brine is introduced into the anolyte compartment of the cell and allowed to percolate through the brine-permeable microporous diaphragm into the catholyte compartment. The microporous diaphragm is sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment. When direct current is applied to the cell, halogen gas is evolved at the anode, hydrogen gas is evolved at the cathode, and an aqueous alkali metal hydroxide solution is formed in the catholyte compartment. In the case of aqueous sodium chloride solutions, the halogen produced is chlorine and the alkali metal hydroxide formed is sodium hydroxide. Catholyte liquor comprising alkali metal hydroxide and unconverted brine is removed from the catholyte compartment of the cell as cell liquor product.
A by-product produced in the aforedescribed electrolysis of aqueous sodium chloride solutions that is observed in the cell liquor product is chlorate ion (ClO3−) e.g., sodium chlorate. The presence of chlorate ion in the cell liquor affects the alkali metal hydroxide, e.g., sodium hydroxide, product quality, and also is corrosive to downstream processing equipment. Some commercial applications for alkali metal hydroxide require very low levels of chlorate ion. It is desirable, therefore, to minimize the amount of chlorate ion produced in a chlor-alkali electrolytic cell.